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<h4 class="subsection">24.3.3 Interaction of <code>signal</code> and <code>sigaction</code></h4>

<p>It's possible to use both the <code>signal</code> and <code>sigaction</code>
functions within a single program, but you have to be careful because
they can interact in slightly strange ways.

   <p>The <code>sigaction</code> function specifies more information than the
<code>signal</code> function, so the return value from <code>signal</code> cannot
express the full range of <code>sigaction</code> possibilities.  Therefore, if
you use <code>signal</code> to save and later reestablish an action, it may
not be able to reestablish properly a handler that was established with
<code>sigaction</code>.

   <p>To avoid having problems as a result, always use <code>sigaction</code> to
save and restore a handler if your program uses <code>sigaction</code> at all. 
Since <code>sigaction</code> is more general, it can properly save and
reestablish any action, regardless of whether it was established
originally with <code>signal</code> or <code>sigaction</code>.

   <p>On some systems if you establish an action with <code>signal</code> and then
examine it with <code>sigaction</code>, the handler address that you get may
not be the same as what you specified with <code>signal</code>.  It may not
even be suitable for use as an action argument with <code>signal</code>.  But
you can rely on using it as an argument to <code>sigaction</code>.  This
problem never happens on the GNU system.

   <p>So, you're better off using one or the other of the mechanisms
consistently within a single program.

   <p><strong>Portability Note:</strong> The basic <code>signal</code> function is a feature
of ISO&nbsp;C<!-- /@w -->, while <code>sigaction</code> is part of the POSIX.1 standard.  If
you are concerned about portability to non-POSIX systems, then you
should use the <code>signal</code> function instead.

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